BASIC COMPONENTS OF A BACTERIAL CELL

• Be aware that different strains of bacteria have special anatomical and physiological traits, which we address elsewhere as these features relate to infectious disease.
• Cell wall
• Plasmic/cytoplasmic membrane is deep to the cell wall.
  – It has an area of infolding. The membrane comprises a lipid bilayer that serves multiple functions, including transport of molecules into the cell, secretion of toxins and enzymes, and energy generation.
• Cytoplasm
  – Ribosomes are sites of protein synthesis.
• Nucleoid is the region with bacterial DNA; there is no nuclear membrane.
• Outer capsule, which most often comprises gelatinous polysaccharides.
  – Variable; When present, the capsule contributes to the serologic type and enhances virulence.
• Pili, aka, fimbriae, which are short filaments found primarily on gram-negative strains.
  – Variable; When present, they participate in bacterial attachment to host cells.
  – The sex pilus attaches donor and recipient bacteria during conjugation.
• Flagella
  – Variable; when present, propels the cell; the number of flagella varies.

STAINING

• Most bacteria associated with infectious disease can be categorized as gram-positive or gram-negative, which refers to whether they retain crystal violet stain, and depends on the composition of their cell walls.

Gram-positive

• In a microscopic sample, we can see the bright purple stain – think Purple Positive.
• Cell wall has thick layer of peptidoglyclan (aka, murein and mucopeptide)
  – It comprises a network of sugars and amino acids, and is the target of some antibiotic drugs.
  – We indicate two acids that are present in the cell wall: teichoic acids, which attach to the peptidoglycan, and, lipoteichoic acid, which are attached to the cytoplasmic membrane.
  – These surface acids contribute to the cell wall structure and charge.

Gram-negative

• In a microscopic sample, gram negative bacteria appear reddish-pink.
• They have a thin peptidoglycan layer, which is covered by the outer membrane.
• The outer membrane is unique to gram negative bacteria and comprises lipopolysaccharide, aka, endotoxin.
  – Endotoxin contributes to disease symptoms such as fever and shock.

Other stain types

• Gram-staining is not appropriate for all bacterial strains.
• We show an image of mycobacteria, which does not have a cell wall and is visualized with acid-fast staining methods.
• We show Treponema pallidum, which is a bacterial strain that has very thin cell wall but can be seen with dark-field microscopy, as in our image, or fluorescent antibodies.
• Some strains, such as the intracellular Chlamydiae, can be seen with hematoxylin and eosin (H&E) staining.

MORPHOLOGY

• The bacterial cell wall contributes to its morphology, which is also used for classification purposes.

Cocci – Spherical

• Clusters
  – Ex: Staphylococcus species
• Chains
  – Ex: Streptococcus pyogenes
• Pairs
  – Ex: Pointed, like Streptococcus pneumoniae
  – Ex: Coffee-bean shaped, like Neisseria gonorrhoeae.

Bacilli – Rods

• Rectangular ends
  – Ex: Bacillus anthracis
• Rounded ends
  – Ex: Salmonella
• Club-shaped
  – Ex: Corynebacterium diptheriae (“Coryne” means “club”)
• Fusiform-shaped
  – Ex: Fusobacterium nucleatum (associated with periodontal disease)
• Bent/comma-shaped
  – Ex: Desulfovibrio desulfuricans (associated with the gut)

Coccobacilli – Intermediate

• Short, rounded rods
  – Ex: Coxiella burnetii (Q fever)

Spirochetes – Spirals

• Ex: Treponema pallidum (Syphilis)

Be aware that there is intertextual variation in regards to bacterial classification schema, and some authors describe more or fewer morphological types.